I finally implemented the local variable usage tracking for subroutines in the verifier. In this instance Microsoft certainly did learn from Sun's mistakes. The JVM verifier is ridiculously complex because of two features:

jsr & ret instructions (subroutines within a method)

untyped local variables

The CLR lacks both of those features. The reason Sun introduced subroutines is because code in a finally block needs to run both in the normal case and in the exception case, there are two ways you can compile this (Java's bytecode has no concept of finally blocks): code duplication or the subroutine construct. Microsoft solved this by explicitly supporting finally blocks in MSIL, a far more elegant solution. Having typed local variables really doesn't have any downside (except that you have slightly more metadata to carry around).

I did some more work on Throwable. In my previous post I forgot to mention that virtual methods in Throwable wheren't overidable yet (because they have no equivalent in System.Exception, I cannot reuse those vtable slots, like I do for System.Object). I implemented a mechanism to support those now. Every direct subclass of Throwable is converted into a .NET subtype of System.Exception and it implements the Throwable$VirtualMethods interface. Each virtual invocation of a virtual method in Throwable is redirected to a static method in the class Throwable$VirtualMethodsHelper that checks if the type implements the Throwable$VirtualMethods interface, if it does, the method is routed there, if it doesn't the method is routed to the C# implementation of the Throwable method in ExceptionHelper.

BTW, the reason that the static helper methods are in a separate class, is because Reflection.Emit (incorrectly) doesn't allow the definition of static methods in interfaces.

The cool part of all this is that all of this virtual method handling is generic and completely based on the remappings defined in map.xml.

I also implemented the 1.4 stack trace handling, so Throwable is now fully functional.

BTW#2, there is also an alternative way I could have handled the virtual methods, by making Throwable a subtype of System.Exception. I haven't figured out the pros and cons of this, so may be I'll do the alternative implementation in a few days to see if that is a more elegant way of doing it.

Throwable.getMessage(), Throwable.getCause() & Throwable.toString() all work, and are fully compatible with J2SE 1.4. Additionally, the message is stored as the System.Exception.Message property, and when the exception object was constructed with a cause, the System.Exception.InnerException property is set.

Stuart commented on the previous item: ...That leaves #2, which seems hard to me. At the very least, it seems like it would require changes to the Java language compiler to interpret (at least metadata from) .NET classes and allow for those to be in the CLASSPATH at compilation time. Is that something you're interested in pursuing as part of IKVM, or do you regard it as a separate problem?

The way I plan to go about this, is to create a tool that generates jar files from .NET assemblies. The jar will contain all the public classes & interfaces from the .NET assembly, and all the methods will appear to be native. Standard Java compilers can then be used to compile against .NET code. The IK.VM will be aware of these special classes (they will contain an attribute to specify the fully qualified .NET type name) and reroute all access to these wrappers to the real .NET types. I haven't built any of this, but I don't expect any problems.

I changed the Java to native code transition from my own MC++ marshaling to use the calli instruction. The code is much more elegant now, not to mention much faster. My database app now performs on a par with JDK 1.1. Not bad at all.

Also, I implemented support for interfaces that "override" java.lang.Object methods and I remapped java.lang.Comparable to System.IComparable. Both of these features use the same underlying mechanism, i.e. the ability to provide explicit interface implementations that redirect to virtual methods with a different name.

Updated the snapshot. Many fixes to the verifier and compiler and I also implemented a few more "native" methods. My company's database server product now works (at least partially), which is a pretty exciting step. It uses JNI to do its file I/O (because it runs on JDK 1.1), so it runs a little slower than under JDK 1.1, but hopefully that will improve once I've optimized the JNI method calling implementation.

Just for fun, I tried to run a FFT program from the IBM Ninja (Numerically Intensive Java) package. After implementing a few more native methods, it ran, but to my surprise it ran about as fast as under JDK 1.4 (with Sun's Hotspot VM). Not bad for totally unoptimized code

The cfft.java source that I ran can be viewed here, the resulting executable (in a zip) can be found here and I've updated the project snapshot. For the array package, follow the Ninja link.

Yesterday, Microsoft released J#. When I first looked at it yesterday, I was happy to find that they fixed the huge flaw that existed in beta 2. In the final version they have made java.lang.Object and java.lang.String aliases for System.Object and System.String. So they no longer require a "VerifierFix" to convert System.Object references to java.lang.Object references. Even so, they've decided to remove the AllowPartiallyUntrustedCallers attribute from the J# runtime libraries.

Unfortunately, the rest of the story isn't so positive. In only an hour of trying to compile my Java code, I already encountered three bugs in the compiler and one in the runtime. So I remain very sceptical about the quality of this product.